Abstract: A hammer assembly for attachment to a rotor of a rotary material crusher. The hammer assembly has an adaptor for attaching to the rotor, having a base portion and a hammer mounting portion, with the base portion being attachable to the rotor at the periphery portion thereof, and a hammer for mounting on the adaptor, having at least one impact portion, for crushing material, and a mounting portion for engagement with the hammer mounting portion of the adaptor. The hammer mounting portion of the adaptor includes bearing surfaces facing a direction of impact when crushing material, and the mounting portion of the hammer includes bearing surfaces for mating with the bearing surfaces of the hammer mounting portion of the adaptor. The hammer assembly further includes a retainer rod for preventing demounting of the hammer from the adaptor. The retainer rod passes through an opening formed by a retainer rod groove in the hammer and a retainer rod groove in the adaptor which face each other.